This section of this document is intended to introduce various aspects of the art that may be related to various aspects of the present disclosure described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. That such art is related in no way implies that it is prior art. The related art may or may not be prior art. It should therefore be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the wellbore wall.
For successful oil and gas exploration, it may be useful to have information about the subsurface formations that are penetrated by a wellbore. For example, one aspect of standard formation evaluation relates to the measurements of the reservoir fluid pressure and/or formation permeability, among other reservoir properties. These measurements may be used to predicting the production capacity and production lifetime of a subsurface formation.
One technique for measuring reservoir properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a wireline in electrical communication with a control system disposed on the surface. The tool is lowered into a well so that it can measure formation properties at desired depths. A typical wireline tool may include a probe or other sealing device, such as a pair of packers, that may be pressed against the wellbore wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” Using the probe, a formation tester measures the pressure of the formation fluids, generates a pressure pulse, which is used to determine the formation permeability. The formation tester tool also typically withdraws a sample of the formation fluid that is either subsequently transported to the surface for analysis or analyzed downhole.
In order to use any wireline tool, whether the tool be a resistivity, porosity or formation testing tool, the drill string is usually removed from the well so that the tool can be lowered into the well. This is called a “trip” uphole. Then, the wireline tools may be lowered to the zone of interest. A combination of removing the drill string and lowering the wireline tools downhole are time-consuming measures and can take up to several hours, depending upon the depth of the wellbore. Because of the great expense and rig time required to “trip” the drill pipe and lower the wireline tools down the wellbore, wireline tools are generally used only when additional information about the reservoir is beneficial and/or when the drill string is tripped for another reason, such as changing the drill bit size. Examples of wireline formation testers are described, for example, in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; 5,622,223; 6,719,049 and 7,380,599 all herein incorporated by reference in their entirety.
To avoid or minimize the downtime associated with tripping the drill string, another technique for measuring formation properties has been developed in which tools and devices are positioned near the drill bit in a drilling system. Thus, formation measurements are made during the drilling process and the terminology generally used in the art is “MWD” (measurement-while-drilling) and/or “LWD” (logging-while-drilling). A variety of downhole MWD and LWD drilling tools are commercially available. Further, formation measurements can be made in tool strings which do not have a drill bit but which may circulate mud in the borehole.
MWD typically refers to measuring the drill bit trajectory as well as wellbore temperature and pressure, while LWD typically refers to measuring formation parameters or properties, such as resistivity, porosity, permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, facilitates making decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process. While LWD and MWD have different meanings to those of ordinary skill in the art, that distinction is not germane to this disclosure, and therefore this disclosure does not distinguish between the two terms.
Formation evaluation tools capable of performing various downhole formation tests typically include a small probe and/or pair of packers that can be extended from a drill collar to establish hydraulic coupling between the formation and sensors and/or sample chambers in the tool. Some tools may use a pump to actively draw a fluid sample out of the formation so that it may be stored in a sample chamber in the tool for later analysis. Such a pump may be powered by a generator in the drill string that is driven by the mud flow down the drill string. Examples of LWD formation testers are described, for example, in U.S. Pat. App. Pub. Nos. 2008/0156486 and 2009/0195250 all herein incorporated by reference in their entirety.